Radiation detector

ABSTRACT

A radiation detector comprises an electrode structure, a planarising layer being disposed over the electrode structure and a protective stack which covers the planarising layer. The planarising layer evens out substantial differences between levels of the electrode structure above the substrate on which the electrode structure is disposed. Consequently, cracks, weak spots and other defects in the protective stack are to a large extent avoided. Because the planarising layer covers essentially the entire electrode structure, practically all sources of defects, notably cracks, in the protective stack are avoided.

The invention relates to a radiation detector having an electrodestructure. Such a radiation detector is known from the U.S. Pat. No.5,777,355.

The known radiation detector is a radiation imager. The electrodestructure of the known radiation imager includes photosensitive elementsand metal bottom contacts which are provided on a substrate. Thephotosensitive elements, notably photodiodes convert incident radiationinto electric charges, which are collected at the bottom metal contacts.A common electrode is often patterned into lines or stripes extendingover the radiation imager. In the cited US-patent it is recognised thatcharge leakage is a critical factor in the performance of thephotodiodes. There are two significant components of charge leakage,viz. area leakage and sidewall leakage. In particular in smaller diodesin which the area of the sidewalls are relatively large with respect tothe overall area of the photodiode, sidewall leakage constitutes theprimary source of leakage. Further, the cited US-patent mentions thatdegradation due to exposure to moisture can make sidewall leakage asignificant leakage source. In the known radiation imager, an attempt ismade to reduce leakage by applying a polyimide over an inorganicmoisture barrier over the upper surface of the photosensitive elements.Still, defects in the moisture barrier layer enable moisture to enterthe polyimide and to then spread out to the photodiodes and affectingsidewall leakage. The cited US-patent mentions the use of polymer bridgemembers that are positioned between the common electrodes and thephotodiodes, so as to form a bridge island between adjacent photodiodes.Individual polymer bridge members extend only between a pair of adjacentphotodiodes so as to form a bridge island between the pair ofphotodiodes. As a result, any leakage defects, such as that resultingfrom moisture or impurities, only affect the pair of adjacentphotodiodes. The polymer bridge members are typically isolated from oneanother so that adverse leakage in a given bridge island does not affectother photodiode pairs. Accordingly, the polymer bridges of the knownradiation imager aim at limiting the effect of leakage defects notablydue to moisture.

An object of the invention is to provide a radiation detector which isless susceptible to penetration of moisture.

This object is achieved by the radiation detector of the inventioncomprising

-   -   an electrode structure    -   a planarising layer being disposed over the electrode structure        and    -   a protective stack which covers the planarising layer.

The radiation detector of the invention comprises an electrode structurethat includes photosensitive elements, such as photodiodes which convertincident radiation into electric charges. Further, the electrodestructure involves collecting electrodes on which the electric chargegenerated in the photosensor elements are stored. In the electrodestructure, read lines are provided to read out the electric charges fromthe collecting electrodes. To this end the collecting electrodes areswitchably coupled to read lines by way of switching elements such as(thin-film) transistors or switching diodes. In order to control theswitching elements, addressing lines are provided in the electrodestructure. Addressing signals are sent over the addressing lines to openand close the switching elements.

The planarising layer evens out substantial differences between levelsof the electrode structure above the substrate on which the electrodestructure is disposed. This planarising is achieved in that the topsurface of the planarising layer, i.e. the surface facing away from theelectrode structure is at substantially a smooth and flat surface. Thisis achieved in that the planarising layer has a local thickness that isless at portions above thicker portions of the electrode structure andthe local thickness is larger at portions above thinner portions of theelectrode structure and still thicker above places where the electrodestructure is absent. In this way differences in thickness betweenvarious portions of the electrode structure are compensated at the topsurface of the planarising layer. Consequently, the planarising layerhas a top surface that is smooth and substantially void of anysubstantial level differences. Good results are achieved when the topsurface of planarising layer has local corrugations that subtend anangle in the range of 0-45°, and preferably in the range of 0-30°.

The present invention is based on an insight that charge leakage mayoccur due to defects such as cracks in the protective stack and thesecracks may occur at places where there are edges in the electrodestructure, not only at the photosensitive elements, such as photodiodes,but also at edges of collecting electrodes, switching elements such astransistors, read lines and addressing lines. Further, preferably theplanarising layer covers essentially the entire electrode structure.This is easily achieved by a planarising layer that continuously extendsover the electrode structure. Then the protective stack is disposed overthe top surface of the planarising layer that is opposite the electrodestructure. Consequently, cracks, weak spots and other defects in theprotective stack are to a large extent avoided. Because the planarisinglayer covers essentially the entire electrode structure, practically allsources of defects, notably cracks, in the protective stack are avoided.At least substantially more causes of defects are avoided than what isachieved in the known radiation detector. In practice the density ofremaining defects in the protective stack is far less, e.g. less than apercent, than the defect density in the electrode structure. Hence, itis achieved that the defect density of the electrode structure is thedominant source of defects in the radiation detector of the invention.In the radiation detector of the invention, the defect density issuppressed close to or even below the measurement threshold.Accordingly, the protective stack being disposed over the planarisinglayer very effectively keeps out moisture out of the electrode structureand notably away from any photosensitive elements. The protective stackmay be formed as a single protective layer or as a package of severallayers.

These and other aspects of the invention will be further elaborated withreference to the embodiments defined in the dependent Claims.

A polymer layer is quite suitable to be employed as the planarisinglayer. Good results are achieved when an epoxy polymer layer isemployed. The viscosity of this epoxy polymer layer is low enough toachieve a planar, flat and smooth top surface. Further, the epoxypolymer layer is easy to pattern, for example by way of lithography orby way of printing. Accordingly, the deposition of the planarising epoxypolymer layer involves a simple process step, so that the radiationdetector according to the invention is easy to manufacture. The epoxypolymer layer has a high optical transparency. The epoxy layer is forexample designed to have an optical refractive index that is equal to orvery close the refractive index of glass. Accordingly, the planarisingepoxy polymer layer does not give rise to optical disturbances in theradiation detector of the invention. Further, the epoxy polymer layer iselectrically very well isolating and remains electrically neutral.Accordingly, the planarising epoxy polymer layer does not give rise toelectrical disturbances in the radiation detector of the invention,notably piezo-electrical effects are avoided.

In a preferred embodiment the protective stack comprises a moistureresistant layer and an outer cover. For example the moisture resistantlayer includes a silicon nitride, silicon oxide or silicon oxynitride.Also a multilayer of several silicon nitride, silicon oxide or siliconoxynitride layers may be used as the moisture resistant layer. Excellentprotection a against moisture is achieved by the moisture resistantlayer of these materials and having a very smooth topography. This verysmooth topography is achieved as the top surface of the planarisinglayer is very level. For example when the moisture resistant layer has athickness of about 1 μm, then the local corrugations of the planarisinglayer should subtend locally an angle of less than 45°. When thethickness of the moisture resistant layer is less, say 0.2 μm, thenlocal corrugations of the planarising layer are required to subtendlocally an even smaller angle e.g. of less than 30°.

Additionally, the protective stack includes the outer cover in the formof a polymer layer on top of the moisture resistant layer whichmechanically protects the radiation detector. In particular the outercover protects the layers underneath the outer cover against scratchesso that penetration of moisture to the electrode structure is avoided.Preferably an acrylate outer cover is employed. The acrylate outer covercan easily be designed to be sufficiently hard to provide goodprotection against scratches.

A further preferred embodiment of the radiation detector includes aconversion layer. The conversion layer converts incident radiation intosecondary radiation of a lower energy. For example, the conversion layeris chosen such that the energy (or wavelength) of the secondaryradiation is in a range for which the photosensitive elements of in theelectrode structure have a high sensitivity. Notably, a scintillatorlayer, such as a CsI:Tl may be employed as a conversion layer to convertincident x-radiation into green light. Photosensor elements in the formof Si pin-diodes appear to have a high sensitivity for the green lightfrom the scintillator layer. According to the invention, the hygroscopicproperty of the scintillator layer does not give rise to defects in theelectrode structure because moisture is very effectively kept out of theelectrode structure by the moisture protective layer. Notably,electrolytic corrosion of the electrode structure is avoided becausemoisture cannot reach the electrode structure, even in spite of thehygroscopic scintillator layer.

The conversion layer can be disposed, e.g. on the moisture resistivelayer of the protective stack. In an alternative embodiment of theradiation detector of the invention, the conversion layer is providedbetween the planarising layer and the protective stack.

Preferably, the radiation detector of the invention is an x-raydetector. The x-ray detector of the invention derives an electronicimage signal from an x-ray image received by the x-ray detector of theinvention.

The invention relates to an electronic device as defined in Claim 6.This electronic device may concern an array photo detector or a displaydevice such as a poly-LED or an organic-LED. These electronic devices ofthe invention are hardly or not at all susceptible to detrimentaleffects of moisture, because moisture is very well kept away fromsensitive portions of the electrode structure of these electronicdevices.

These and other aspects of the invention will be elucidated withreference to the embodiments described hereinafter and with reference tothe accompanying drawing wherein

FIG. 1 shows a circuit diagram of a sensor matrix 1 incorporated in anx-ray detector according to the invention.

FIG. 2 shows a cross sectional view of an x-ray detector according tothe invention.

FIG. 1 shows a circuit diagram of a sensor matrix 1 incorporated in anx-ray detector according to the invention. The sensor matrixincorporates a plurality of sensor elements arranged in a matrix. Foreach pixel in the x-ray image there is provided a sensor element 21which comprises a photosensitive element 22, a collecting capacitance 23and a switching element 4. Electric changes are derived from incidentx-rays by the photosensitive element 22, which electric charges arecollected by the collection capacitance 23. The collecting electrodes 3form part of respective collecting capacitances 23. For each column ofsensor elements there is provided a respective read-lines 19 and eachcollecting capacitance 23 is coupled to its respective read-line 19 byway of its switching element 4. Although as an example FIG. 1 shows only3×3 sensor elements, in a practical embodiment a much larger number ofsensor elements say 2000×2000, is employed.

In order to read-out the collected electric charges the relevantswitching elements 4 are closed so as to pass electric charges downrespective read-lines. Separate read-lines 19 are coupled to respectivehighly sensitive output amplifiers 24 of which the output signals aresupplied to a multiplex circuit 25. The electronic image signal iscomposed from the output signals by the multiplex circuit 25. Theswitching elements 4 are controlled by means of a row-driver circuit 26which is coupled to the switching elements for each row by means ofaddressing lines 27. The switching elements 4 are preferably formed asthin-film transistors (TFT) of which the drain contact is connected to arelevant read-line, the source contact is connected to the relevantcollecting electrode and the gate contact is coupled to the relevantaddressing line. The multiplex circuit supplies the electronic imagesignal e.g. to a monitor 28 on which the image information of the x-rayimage is then displayed or the electronic image signal may be suppliedto an image processor 29 for further processing.

FIG. 2 shows a cross sectional view of an x-ray detector according tothe invention. FIG. 3 notably shows in cross-sectional view a thin-filmstructure of a sensor matrix incorporated in the x-ray detector of thex-ray examination apparatus according to the invention. On the substrate30 there are disposed the thin-film transistors 4 and photodiodes 22,which form the photo-electric elements. Notably, instead of photodiodesthere may be employed semiconductor photoconducting elements orphototransistors as the photo-electric elements. In particularphotodiodes have a simple structure and are therefore easy tomanufacture. The photodiodes convert incident radiation such as light orinfrared radiation into electric charges. In particular a pin-diodestructure is suitable to form such a photodiode. The thin-filmtransistors 4 form switching elements which couple the photodiodes 22 torespective read-lines 19.

On top of the electrode structure that is formed by e.g. thephotosensitive elements 22, read lines 19, addressing lines 24,switching elements 4, collecting electrodes 3, gate contacts 35, theplanarising layer 101 is deposited. The epoxy polymer planarising layerat its top surface 102 any level differences at among the variouscomponents of the electrode structure are to a large extent levelledoff. The planarising layer 101 is covered by the protective stack whichincludes the moisture resistant layer 103 and the acrylate outer cover105. In the protective stack the conversion layer 104 in the form of ascintillation layer of e.g. CsI:Tl is integrated.

Such a scintillation layer converts incident x-rays into green light forwhich the photodiodes are substantially sensitive. Preferably, theCsI:Tl is deposited in the form of columnar crystals, groups of whicheffectively form light-guides. Such groups of columnar crystals areseparated by cracks that are distributed preferably at about 200-400cracks per centimetre. Typically the thickness of the CsI:Tl layer is inthe range 500-1000 μm.

1. A radiation detector comprising: an electrode structure; aplanarising layer being disposed over the electrode structure; and aprotective stack which covers the planarising layer, wherein theprotective stack has a moisture resistant layer and a conversion layer,wherein the conversion layer converts incident radiation into secondaryradiation, and wherein the moisture resistant layer is positionedbetween the conversion layer and the planarising layer, wherein theelectrode structure and the planarising layer are positioned along asubstrate that is substantially flat, and wherein the planarising layeris in direct contact with the electrode structure and the substrate. 2.A radiation detector as claimed in claim 1, wherein the planarisinglayer is formed as a polymer layer.
 3. A radiation detector as claimedin claim 1, wherein the protective stack includes an outer cover.
 4. Theradiation detector of claim 3, wherein the planarising layer is apolymer layer.
 5. A radiation detector as claimed in claim 1, whereinthe conversion layer is a scintillation layer formed of columnarcrystals.
 6. A radiation detector as claimed in claim 5, wherein thescintillation layer is formed from CsI:Tl.
 7. The radiation detector ofclaim 1, wherein the moisture resistant layer is in direct contact withthe planarising layer.
 8. An electronic device comprising: a substrate;an electrode structure disposed on the substrate; a planarising layerbeing disposed over the electrode structure and substrate; and aprotective stack which covers the planarising layer, wherein theprotective stack has a moisture resistant layer and a conversion layer,wherein the conversion layer converts incident radiation into secondaryradiation, wherein the moisture resistant layer is positioned betweenthe conversion layer and the planarising layer, wherein the conversionlayer is in direct contact with the moisture resistant layer, andwherein the substrate is substantially flat.
 9. The electronic device ofclaim 8, wherein the planarising layer is formed as a polymer layer. 10.The electronic device of claim 8, wherein the conversion layer is ascintillation layer formed of columnar crystals.
 11. The electronicdevice of claim 10, wherein the scintillation layer is formed fromCsI:Tl.
 12. The electronic device of claim 8, wherein the moistureresistant layer is in direct contact with the planarising layer.
 13. Theelectronic device of claim 8, wherein the planarising layer is in directcontact with the electrode structure and the substrate, and wherein theplanarising layer is a polymer layer.
 14. An electronic devicecomprising: a substrate: an electrode structure disposed on thesubstrate; a planarising layer being disposed over the electrodestructure and substrate; and a protective stack which covers theplanarising layer, wherein the protective stack has a moisture resistantlayer and a conversion layer, wherein the conversion layer convertsincident radiation into secondary radiation, wherein the moistureresistant layer is positioned between the conversion layer and theplanarising layer, wherein the substrate is substantially flat, andwherein the protective stack includes an outer cover.